G. Israelian

Chemical abundances of 1111 FGK stars from the HARPS GTO planet search program - Galactic stellar populations and planets

Context. We performed a uniform and detailed abundance analysis of 12 refractory elements (Na, Mg, Al, Si, Ca, Ti, Cr, Ni, Co, Sc, Mn, and V) for a sample of 1111 FGK dwarf stars from the HARPS GTO planet search program. Of these stars, 109 are known to harbor giant planetary companions and 26 stars are exclusively hosting Neptunians and super-Earths. Aims. The two main goals of this paper are to investigate whether there are any differences between the elemental abundance trends for stars of different stellar populations and to characterize the planet host and non-host samples in terms of their [X/H]. The extensive study of this sample, focused on the abundance differences between stars with and without planets will be presented in a parallel paper. Methods. The equivalent widths of spectral lines were automatically measured from HARPS spectra with the ARES code. The abundances of the chemical elements were determined using an LTE abundance analysis relative to the Sun, with the 2010 revised version of the spectral synthesis code MOOG and a grid of Kurucz ATLAS9 atmospheres. To separate the Galactic stellar populations we applied both a purely kinematical approach and a chemical method. Results. We found that the chemically separated (based on the Mg, Si, and Ti abundances) thin- and thick disks are also chemically disjunct for Al, Sc, Co, and Ca. Some bifurcation might also exist for Na, V, Ni, and Mn, but there is no clear boundary of their [X/Fe] ratios. We confirm that an overabundance in giant-planet host stars is clear for all studied elements.We also confirm that stars hosting only Neptunian-like planets may be easier to detect around stars with similar metallicities than around non-planet hosts, although for some elements (particulary α-elements) the lower limit of [X/H] is very abrupt.

Oxygen Abundances in Unevolved Metal-poor Stars from Near-Ultraviolet OH Lines

We have performed a detailed oxygen abundance analysis of 23 metal-poor (-3.0 < [Fe/H] < -0.3) unevolved halo stars and one giant through the OH bands in the near UV, using high-resolution echelle spectra. Oxygen is found to be overabundant with respect to iron in these stars, with the [O/Fe] ratio increasing from 0.6 to 1 between [Fe/H] = -1.5 and -3.0. The behavior of the oxygen overabundance with respect to [Fe/H] is similar to that seen in previous works based on O I IR triplet data. Contrary to the previously accepted picture, our oxygen abundances, derived from low-excitation OH lines, agree well with those derived from high-excitation lines of the triplet. For nine stars in common with Tomkin et al. we obtain a mean difference of 0.00 ± 0.11 dex with respect to the abundances determined from the triplet using the same stellar parameters and model photospheres. For four stars in our sample we have found measurements of the [O I] λ6300 line in the literature, from which we derive oxygen abundances consistent (average difference 0.09 dex) with those based on OH lines, showing that the long-standing controversy between oxygen abundances from forbidden and permitted lines in metal-poor unevolved stars can be resolved. Our new oxygen abundances show a smooth extension of the Edvardsson and coworkers [O/Fe] versus metallicity curve to much lower abundances, with a slope of -0.31 ± 0.11 (taking into account the error bars in both oxygen abundances and metallicities) in the range -3 < [Fe/H] < -1. The extrapolation of our results to very low metallicities indicates that the first Type II supernovae in the early Galaxy provided oxygen to iron ratios of [O/Fe] 1. The oxygen abundances of unevolved stars, when compared with values in the literature for giants of similar metallicity, imply that the latter may have suffered a process of oxygen depletion. As a result, unevolved metal-poor stars shall be considered better tracers of the early evolution of oxygen in the Galaxy. The higher [O/Fe] ratios we find in dwarfs has an impact on the age determination of globular clusters, suggesting that current age estimates have to be reduced by about 1-2 Gyr.

A new code for automatic determination of equivalent widths: Automatic Routine for line Equivalent widths in stellar Spectra (ARES)

Aims. We present a new automatic code (ARES) for determining equivalent widths of the absorption lines present in stellar spectra. We also describe its use for determining fundamental spectroscopic stellar parameters. Methods. The code is written in C++ based on the standard method of determining EWs and is available for the community. The code automates the manual procedure that the users normally carry out when using interactive routines such as the splot routine implemented in IRAF. Results. We test the code using both simulated and real spectra with different levels of resolution and noise and comparing its measurements to the manual ones obtained in the standard way. The results shows a small systematic difference, always below 1.5 mA. This can be explained by errors in the manual measurements caused by subjective continuum determination. The code works better and faster than others tested before.

Spectroscopic stellar parameters for 582 FGK stars in the HARPS volume-limited sample - Revising the metallicity-planet correlation

To understand the formation and evolution of solar-type stars and planets in the solar neighborhood, we need to obtain their stellar parameters with high precision. We present a catalog of precise stellar parameters for low-activity FGK single stars in a volumelimited sample followed by the HARPS spectrograph in the quest to identify extra-solar planets. The spectroscopic analysis was completed assuming LTE with a grid of Kurucz atmosphere models and using the ARES code to perform an automatic measurement of the line equivalent widths. The results are compared with different independent methods and also with other values found in the literature for common stars. Both comparisons are consistent and illustrate the homogeneity of the parameters derived by our team. The derived metallicities of this sample reveal a somewhat different distribution for the present planet hosts, but still indicates the already known higher frequency of planets observed for the more metal-rich stars. We combine the results derived in this sample with the one from the CORALIE survey to present the largest homogeneous spectroscopic study of the metallicity-giant-planet relation using a total of 1830 stars.

Chemical abundances of 451 stars from the HARPS GTO planet search program - Thin disc, thick disc, and planets

We present a uniform study of the chemical abundances of 12 elements (Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Na, Mg, and Al) derived from the spectra of 451 stars observed as part of one of the HARPS GTO planet search programs. Sixty eight of these are planetbearing stars. The main goals of our work are: i) the investigation of possible differences between the abundances of stars with and without planets; ii) the study of the possible differences in the abundances of stars in the thin and the thick disc. We confirm that there is a systematically higher metallicity in planet host stars, when compared to non planet-hosts, common to all studied species. We also found that there is no difference in the galactic chemical evolution trends of the stars with and without planets. Stars that harbour planetary companions simply appear to be in the high metallicity tail of the distribution. We also confirm that Neptunian and super-Earth class planets may be easier to find at lower metallicities. A statistically significative abundance difference between stars of the thin and the thick disc was found for [Fe/H] < 0. However, the populations from the thick and the thin disc cannot be clearly separated.

Enhanced lithium depletion in Sun-like stars with orbiting planets

The surface abundance of lithium on the Sun is 140 times less than the protosolar value, yet the temperature at the base of the surface convective zone is not hot enough to burn—and hence deplete—Li (refs 2, 3). A large range of Li abundances is observed in solar-type stars of the same age, mass and metallicity as the Sun, but such a range is theoretically difficult to understand. An earlier suggestion that Li is more depleted in stars with planets was weakened by the lack of a proper comparison sample of stars without detected planets. Here we report Li abundances for an unbiased sample of solar-analogue stars with and without detected planets. We find that the planet-bearing stars have less than one per cent of the primordial Li abundance, while about 50 per cent of the solar analogues without detected planets have on average ten times more Li. The presence of planets may increase the amount of mixing and deepen the convective zone to such an extent that the Li can be burned.

Evidence for planet engulfment by the star HD82943

Current models of the evolution of the known extrasolar planetary systems need to incorporate orbital migration and/or gravitational interactions among giant planets to explain the presence of large bodies close to their parent stars. These processes could also lead to planets being ingested by their parent stars, which would alter the relative abundances of elements heavier than helium in the stellar atmospheres. In particular, the abundance of the rare 6Li isotope, which is normally destroyed in the early evolution of solar-type stars but preserved intact in the atmospheres of giant planets, would be boosted substantially. 6Li has not hitherto been observed reliably in a metal-rich star, where metallicity refers to the total abundance of elements heavier than helium. Here we report the discovery of 6Li in the atmosphere of the metal-rich solar-type star HD82943, which is known to have an orbiting giant planet. The presence of 6Li can probably be interpreted as evidence for a planet (or planets) having been engulfed by the parent star.

CHEMICAL CLUES ON THE FORMATION OF PLANETARY SYSTEMS: C/O VERSUS Mg/Si FOR HARPS GTO SAMPLE

Theoretical studies suggest that C/O and Mg/Si are the most important elemental ratios in determining the mineralogy of terrestrial planets. The C/O ratio controls the distribution of Si among carbide and oxide species, while Mg/Si gives information about the silicate mineralogy. We present a detailed and uniform study of C, O, Mg, and Si abundances for 61 stars with detected planets and 270 stars without detected planets from the homogeneous high-quality unbiased HARPS GTO sample, together with 39 more planet-host stars from other surveys. We determine these important mineralogical ratios and investigate the nature of the possible terrestrial planets that could have formed in those planetary systems. We find mineralogical ratios quite different from those of the Sun, showing that there is a wide variety of planetary systems which are not similar to our solar system. Many planetary host stars present an Mg/Si value lower than 1, so their planets will have a high Si content to form species such as MgSiO3. This type of composition can have important implications for planetary processes such as plate tectonics, atmospheric composition, or volcanism.

SWEET-Cat: A catalogue of parameters for Stars With ExoplanETs - I. New atmospheric parameters and masses for 48 stars with planets

Context. Thanks to the importance that the star-planet relation has to our understanding of the planet formation process, the precise determination of stellar parameters for the ever increasing number of discovered extra-solar planets is of great relevance. Furthermore, precise stellar parameters are needed to fully characterize the planet properties. It is thus important to continue the efforts to determine, in the most uniform way possible, the parameters for stars with planets as new discoveries are announced. Aims. In this paper we present new precise atmospheric parameters for a sample of 48 stars with planets. We then take the opportunity to present a new catalogue of stellar parameters for FGK and M stars with planets detected by radial velocity, transit, and astrometry programs. Methods. Stellar atmospheric parameters and masses for the 48 stars were derived assuming local thermodynamic equilibrium (LTE) and using high-resolution and high signal-to-noise spectra. The methodology used is based on the measurement of equivalent widths for a list of iron lines and making use of iron ionization and excitation equilibrium principles. For the catalogue, and whenever possible, we used parameters derived in previous works published by our team, using well-defined methodologies for the derivation of stellar atmospheric parameters. This set of parameters amounts to over 65% of all planet host stars known, including more than 90% of all stars with planets discovered through radial velocity surveys. For the remaining targets, stellar parameters were collected from the literature. Results. The stellar parameters for the 48 stars are presented and compared with previously determined literature values. For the catalogue, we compile values for the effective temperature, surface gravity, metallicity, and stellar mass for almost all the planet host stars listed in the Extrasolar Planets Encyclopaedia. This data will be updated on a continuous basis. The compiled catalogue is available online. The data can be used for statistical studies of the star-planet correlation, as well as for the derivation of consistent properties for known planets.

Oxygen in the Very Early Galaxy

Oxygen abundances in a sample of ultra-metal-poor subdwarfs have been derived from measurements of the oxygen triplet at 7771-5 A and OH lines in the near-UV performed in high-resolution and high signal-to-noise ratio spectra obtained with WHT/UES, Keck I/HIRES, and VLT/UVES. Our Fe abundances were derived in LTE and then corrected for non-LTE (NLTE) effects. The new oxygen abundances confirm previous findings for a progressive linear rise in the oxygen-to-iron ratio with a slope -0.33 ± 0.02 from solar metallicity to [Fe/H] ~ -3. A slightly higher slope would be obtained if the Fe NLTE corrections were not considered. Below [Fe/H] = -2.5 our stars show [O/Fe] ratios as high as ~1.17 (G64-12), which can be interpreted as evidence for oxygen overproduction in the very early epoch of the formation of the halo, possibly associated with supernova events with very massive progenitor stars. We show that the arguments against this linear trend given by Fulbright & Kraft in 1999, based on the LTE Fe analysis of two metal-poor stars, cannot be sustained when an NLTE analysis is performed. We discuss how the Fulbright & Kraft LTE ionization balance of Fe lines underestimates the gravity of the very metal-poor star BD +23°3130 ([Fe/H] = -2.43) and how this leads to an underestimation of the oxygen abundance derived from the forbidden line. Gravities from Hipparcos appear to be in good agreement with those determined in NLTE, giving higher values than previously assumed, which reduces the discrepancies between the oxygen abundances determined from OH, triplet, and forbidden lines. Using one-dimensional models, our analysis of three oxygen indicators available for BD +23°3130 gives an average [O/Fe] ratio of 0.78. The high oxygen abundances at very low metallicities do not pose a problem to theoretical modeling since there is a range of parameters in the calculations of nucleosynthesis yields from massive stars at low metallicities that can accommodate our results.